r = sqrt(A(5, 5)^2 + A(13, 5)^2);
c = A(5, 5) / r; s = A(13, 5) / r;
G = eye(13);
G(5, 5) = c; G(5, 13) = s; G(13, 5) = -s; G(13, 13) = c;
G1 = G;
A1 = G * A;